Vaso-occlusive devices or implants are used for a wide variety of reasons, including treatment of intra-vascular aneurysms. Commonly used vaso-occlusive devices include soft, helically wound coils formed by winding a platinum (or platinum alloy) wire strand about a “primary” mandrel. The relative stiffness of the coil will depend, among other things, on its composition, the diameter of the wire strand, the diameter of the primary mandrel, and the pitch of the resulting primary windings. The coil is then wrapped around a larger, “secondary” mandrel, and heat treated to impart a secondary shape. For example, U.S. Pat. No. 4,994,069, issued to Ritchart et al., describes a vaso-occlusive coil that assumes a linear, helical primary shape when stretched for placement through the lumen of a delivery catheter, and a folded, convoluted secondary shape when released from the delivery catheter and deposited in the vasculature.
In order to deliver the vaso-occlusive coils to a desired site in the vasculature, e.g., within an aneurismal sac, it is well-known to first position a small profile, delivery catheter or “micro-catheter” at the site using a steerable guidewire. Typically, the distal end of the micro-catheter is provided, either by the attending physician or by the manufacturer, with a selected pre-shaped bend, e.g., 45°, 90°, “J”, “S”, or other bending shape, depending on the particular anatomy of the patient, so that it will stay in a desired position for releasing one or more vaso-occlusive coil(s) into the aneurysm once the guidewire is withdrawn. A delivery or “pusher” wire is then passed through the micro-catheter, until a vaso-occlusive coil coupled to a distal end of the delivery wire is extended out of the distal end opening of the micro-catheter and into the aneurysm. The vaso-occlusive device is then released or “detached” from the end delivery wire, and the delivery wire is withdrawn back through the catheter. Depending on the particular needs of the patient, one or more additional occlusive devices may be pushed through the catheter and released at the same site.
One well-known way to release a vaso-occlusive coil from the end of the pusher wire is through the use of an electrolytically severable junction, which is a small exposed section or detachment zone located along a distal end portion of the pusher wire. The detachment zone is typically made of stainless steel and is located just proximal of the vaso-occlusive device. An electrolytically severable junction is susceptible to electrolysis and disintegrates when the pusher wire is electrically charged in the presence of an ionic solution, such as blood or other bodily fluids. Once the detachment zone exits out of the catheter distal end and is exposed in the vessel blood pool of the patient, a current applied through an electrical contact to the conductive pusher wire completes a circuit with a return electrode, and the detachment zone disintegrates due to electrolysis. Return electrodes include electrodes attached to the patient's skin, conductive needles inserted through the skin at a remote site, and electrodes located on the pusher wire but electrically insulated from the conductive path ending in the detachment zone.
One perceived problem with current vaso-occlusive coil delivery systems is that the detachment zone of the pusher wire bends as the vaso-occlusive coil is pushed through the micro-catheter. Orthogonal forces generated as a stiff pusher wire takes on the shapes of various bends in the micro-catheter may be sufficient to bend the detachment zone. This bending may adversely impact the placement of the embolic coil within the aneurysm and detachment of the embolic coil by electrolysis.
Another perceived problem is that pusher wires tend to have a stiff distal section that complicates accurate placement of the delivery system at the desired location, i.e., a stiff distal section of the pusher wire can cause a pre-shaped micro-catheter to kick back or recoil from the aneurysm upon coil deployment and release.